This invention relates to electron beam furnaces for vacuum refining of metals and metal alloys.
In vacuum refining of metallic materials such as titanium alloy, a feedstock, which may be scrap metal, is supplied to a cold hearth maintained at a vacuum and heated by application of energy from plasma torches or electron beam guns to melt the metal and separate impurities by vaporization, dissolution or gravity. Desired proportions of alloying constituents are also included in the raw material so that, when the molten metal is poured from the hearth into a mold to form an ingot, the ingot has a predetermined alloy composition.
Conventional furnace arrangements, however, present substantial difficulties in the refining of such alloys. Cold hearth furnaces using electron beam energy sources require a high vacuum on the order of 0.1-1 microns Hg in the gun region to prevent rapid deterioration of the cathode and filament in the electron beam guns. When molten metal mixtures are maintained at such high vacuum, however, necessary alloying constituents may be vaporized to an undesired extent, requiring adjustment of the content of those constituents in the raw material supplied to the furnace. Furthermore, in order to attain such high vacuums, substantial degassing times, on the order of five or more hours, are required upon start-up of a furnace from the cold condition. In addition, at such high vacuums, the vaporized constituents or impurities tend to form a loose coating or crust on the interior walls of the furnace and relatively large pieces of the coating may separate from the walls and fall back into the molten material, contaminating it to vary the composition from the desired value and forming undesired inclusions in the cast ingot.
On the other hand, furnaces provided with plasma guns as energy sources are normally operated at higher pressures, such as 100 microns Hg or more, and are less efficient when operated at lower pressures. Because of the higher-pressure conditions prevailing in furnaces using plasma guns as energy sources, refining which requires vaporization of relatively low-volatility impurities is not possible. The higher pressures prevailing in plasma furnaces, however, tend to suppress volatilization of desired allow constituents, thereby avoiding the necessity for adjusting the raw material mixture to compensate for volatilization of components.
Moreover, at pressures above about 100 microns Hg, volatilized materials tend to condense on the walls of the furnace in the form of fine powders, as described, for example, in the Scheller et. al U.S. Pat. No. 3,211,548. The deposited powders can easily be removed from the walls by applying physical agitation, for example, by using vibrators, and they are readily remelted if returned to the molten metal in the hearth so as to eliminate the possibility of undissolved inclusions.
The Hunt U.S. Pat. No. 4,027,722 proposes to take advantage of the desirable aspects of both electron beam furnaces and plasma furnaces by providing successive melting, refining and casting stages which are maintained at different vacuum levels. For this purpose, however, Hunt requires several compartmentalized sections and provides different energy sources such as plasma guns for relatively high-pressure sections and electron beam guns for high-vacuum sections. The Tarasescu et al. U.S. Pat. No. 4,482,376, on the other hand, seeks to provide a plasma gun furnace having the advantages of relatively high vacuum obtained in an electron beam furnace by utilizing a specially-designed large-area plasma gun and operating in the range of 10-100 microns Hg.